Numerous communication protocol standards have developed to enable using the pre-existing plain old telephone system (POTS) infrastructure for carrying digital data. Although the public switched telephone network (PSTN) is digital in nature, the connection between subscribers and the central office which serves as an entry point to the PSTN is analog. As a result, modems are used for bi-directional communication of digital data on the analog channel between the subscriber and the central office. Modems convert the communicated information between the digital and analog domains in accordance with the particular communication protocol.
Some communication protocols are designed to rely on the voiceband region of the analog channel to convey information. As a result, when a subscriber line is in use by such a voiceband modem, the line is unavailable for simultaneous voice communications.
Digital subscriber lines (xDSL) services can provide significantly higher data transmission rates by utilizing communication bandwidth beyond but excluding the voiceband. As a result, xDSL services may simultaneously co-exist with voiceband communications.
Modems or other devices designed for communicating digital data on the analog channel utilize an analog front end for transmitting as well as receiving information from the subscriber line. The analog front end conditions signals communicated to or from the subscriber line before providing the conditioned signal to the subscriber line for transmission or to a digital signal processor for interpretation.
A hybrid circuit, for example, is used to address echoes resulting from using the same two wires for both transmission and reception on the analog channel. One disadvantage of typical hybrid designs is that the high order introduces distortion and noise into the system.
Preferably, a modem has the ability to support the higher data rates of xDSL when available. Due to the geographical limitations on xDSL, voiceband modems are still needed to ensure a reliable means of communication. One risk averse solution implements the functionality of voiceband and xDSL modems through the use of a chipset mounted on a common circuit board.
The combination of the circuitry onto a common board introduces new problems. In particular, the voiceband and xDSL modems rely on clocks of different frequencies which are not multiples of each other. The interaction between two clocks, for example, may result in intermodulation, synchronization problems, or other issues which interfere with the digital signal processor's ability to properly interpret received information.